The present invention relates to a turbo-molecular pump utilizing a magnetic bearing for contactlessly supporting a bell-shaped rotor by a combination of electromagnets and permanent magnets or only by electromagnets.
Though various types of turbo-molecular pumps have been proposed in which a rotor with radial blades is rotationally driven at high speed, such pumps have had various problems relating to the supporting portion (bearing portion) of the rotor on account of the high speed rotation of the rotor.
In a well-known conventional type of turbo-molecular pump in which a rotor is supported by a mechanical bearing which requires lubrication oil, hydrocarbons which have separated from the oil do not flow backward to an intake port while the pump is operating due to the gas current flowing from the intake port to an exhaust port. However, there is a possibility that the hydrocarbons may flow backward by diffusion to reach the intake port during the inoperative state of the pump when the gas current is stopped. The presence of such hydrocarbons is unacceptable in experiments pertaining to surface physics, etc., and this disadvantage has resulted in the restricted range of utilization of the conventional turbo-molecular pump or a need for an expensive valve for preventing reverse gas current flow. Further, the rotational speed of a turbo-molecular pump whose capacity increases according to the rotational speed of the blade, is limited to a certain value since the mechanical bearing wears faster as the rotational speed increases.
Another type of turbo-molecular pump employing a five axes controlled type of magnetic bearing is disclosed in Japanese Publication No. 30998/82 and comprises a rotor supported contactlessly by the use of one axial magnetic bearing and two radius direction magnetic bearings. This pump has an advantage that hydrocarbons do not flow to the intake port side since oil lubrication is not required. However, since five degrees of freedom of the rotor other than the rotation are actively controlled in such a pump, five pairs of controlling loops consisting a position sensor, a gain/phase compensating circuit, an electromagnet driving circuit and an electromagnet are required, which results in increased cost and enlargement in the size of the pump due to a more complicated bearing construction and control circuit as compared with a mechanical bearing system. Further, in this type of pump, though the bearing is free from wear due to the contactless supporting, the service life thereof is restricted by the reliability of the circuit. Therefore, the service life is not so long due to the large number of the circuit elements, and further, the rotational speed cannot be raised higher for fear of centrifugal force acting, during rotation, on an electromagnetic steel plate of weaker material strength, because of an outer-rotor type of turbo-molecular pump. As described above, in this system, the desirable characteristic of the contactless supporting is not sufficiently utilized.
To overcome these disadvantages, another turbo-molecular pump employing a magnetic bearing which is simple in construction and circuit design is disclosed in Japanese Patent Publication No. 12095/81 and in which only the axial direction is actively controlled and the other four degrees of freedom relating to the radius direction are passively controlled. However, it is not practically used for the reasons that when the rotational speed of the rotor is increased beyond the resonance point of the system, the rotor undergoes a whirling motion which lasts too long due to inadequate attenuation in the radius direction.
Further, a tubo-molecular pump comprising a magnetic bearing for the upper part and a mechanical bearing for the lower part is disclosed in U.S. Pat. No. 3,749,528. However, this type of pump does not fully make use of the advantages of a magnetic bearing, such as no need for oil lubrication and complete contactless support of the rotor.